1. Development of Invertebrates Structure
Zoology
Mr. Walker

2. Hierarchy of Developmental Decisions.

3. Fertilization Sperm and egg fuse to create a zygote.
Polyspermy occurs when more than one sperm attempts to fertilize an oocyte.
When a sperm fertilizes an oocyte, an event called “fast block” occurs.
The first sperm with the egg membrane is instantly followed by an electrical potential change in the egg membrane that prevents additional sperm from fusing with the membrane.
Then, the cortical reaction occurs in which thousands of enzyme rich cortical granules, located beneath the egg membrane, fuse with the egg membrane and create an osmotic gradient, causing water to rush into the space, elevating the envelope and lifting away all sperm bound to it. It creates the fertilization membrane to keep extra sperm away from the zygote.

4. Fertilization The union of male and female gametes to form a zygote.
Fertilization accomplishes two things:
it provides for recombination of paternal and maternal genes, thus restoring the original diploid number of chromosomes characteristic of a species.
It activates the egg to begin development.

5. Protostomes and Deuterostomes
Mollusks, annelids, arthropods, and several other phyla represent one line called Protostomes.
Echinoderms, chordates and some other phyla represent Deuterostomes.

6. CLEAVAGE AND EARLY DEVELOPMENT
During cleavage the embryo divides repeatedly to convert the large, unwieldy cytoplasmic mass into a large cluster of small, maneuverable cells called blastomeres.
No growth occurs during this period, only subdivision of mass, which continues until normal somatic cell size is attained.
At the end of cleavage the zygote has been divided into many hundreds or thousands of cells and the blastula stage is formed.

7. Cleavage Protostomes have spiral determinate cleavage.
In spiral cleavage, planes of cell division are diagonal to the vertical axis of the embryo.
Determinate cleavage rigidly casts the developmental fate of each embryonic cell very early.
Deuterostomes have radial, indeterminate cleavage.
Radial cleavage is where the planes are either parallel or perpendicular to the vertical axis of the egg.
Indeterminate cleavage means that each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. (Twins e.g.)

8. Blastulation
Cleavage subdivides the mass of the zygote until a cluster of cells called a blastula (Gr. blastos, germ, ule, little) is formed ( Figure 8.9 ).

9. Symmetry
Radial: Cnidarians and Ctenophores have radial symmetry (radiata).
Bilateral: have a dorsal and ventral side. Have anterior and and posterial side. (bilateria).

10. What Can We Learn From Development?
Biologists study development for different reasons. Some studies focus on understanding how the zygote, a single large cell, can produce the multitude of body parts in an organism.